1. Field of the Invention
This invention relates to novel high temperature superconducting (HTS) metal oxide powders and a process for making same, and more particularly, to high quality HTS powders, typically YBa.sub.2 Cu.sub.3 O.sub.6+x, where x is between 0.5 and 1.0, and a process for making same which uses inexpensive reagents, involves simple unit processes, and can be readily scaled for commercial operations.
2. Background of the Invention
High temperature superconducting materials can be fabricated by a variety of techniques, of which solid state, solvent removal, sol-gel, and coprecipitation are currently the most widely used.
In the solid state technique one starts with oxygen-containing compounds of the desired components. Typically, one would start with oxides, nitrates, or carbonates of Sc, Y or one or more of the rare metals having atomic numbers of from 57 to 71, one or more of the alkaline earth metals, and copper. These solid state compounds are mixed in the desired atomic ratios and ground to a fine powder. The compounds are then reacted by calcining for an extended period at elevated temperatures. The pulverization and calcination steps may be repeated a number of times. The material is then oxygen-annealed and cooled slowly to room temperature.
Solvent removal routes include spray-drying, spray roasting, and freeze drying.
In the sol-gel process an aqueous solution containing the proper ratios of, for example, Y, Ba and Cu nitrates is emulsified in an organic phase and the resulting drops of the discontinuous phase are gelled by the addition of high molecular weight primary amine which serves to extract the nitric acid.
In the coprecipitation process high temperature superconductor precursors are produced by simultaneous precipitation from a solution containing the metal cations. Once the precipitate has been dried, the material can undergo calcination, as in the solid state process.
The coprecipitation technique is considerably less complicated than the sol-gel and the solvent removal processes and can thus be more readily scaled up to commercial scale operations. Coprecipitation methods are attractive for large-scale powder production because of the desirable stoichiometry, homogeneity, and particle size control.
The main advantages of coprecipitation over the solid state technique are that coprecipitation (1) allows for the simultaneous and controlled precipitation of all the components at a fixed ratio, (2) produces a pure, homogeneous, and fine-grained powder, and (3) leads to a reproducible commercial process upon scale-up.
In the coprecipitation process the dissolved cations must dissolve simultaneously in a fixed and constant ratio out of a solution saturated with a precipitating agent. Further, the pH of the solution must be controlled to give product of optimal quality.
A number of coprecipitation techniques have been disclosed in the literature. Most use expensive reagents such as oxalates, alkoxides or tetraammonium hydroxide. For example, U.S. Pat. No. 4,804,649 uses a potassium oxalate precipitating reagent. Similarly, oxalic acid is used in S. Uchida, H. Takagi, K. Kitazawa and S. Tanaka, Jpn. J. Appl. Phys. 26 (1987) L1, H. Ihara, M. Hirabayashi, N. Terada, Y. Kimura, K. Senzaki and M. Tokumoto, Jpn. J. Appl. Phys. 26 (1987) L463, National Institute for Research in Inorganic Materials Research Report of the National Institute for Research in Inorganic Materials No. 49 (1986) p. 4, and K. Kaneko, H. Ihara, M. Hirabayashi, N. Terada and K. Senzaki, Jpn. J. Appl. Phys. 26 (1987) L734.
Oxalates are also used in F. Caillaud, J. Baumard and A. Smith, Mat. Res. Bull., 23 (1988) 1273, and in P. Pramanik, S. Biswas, C. Singh, D. Bhattacharya, T. K. Dey, D. Sen, S. K. Ghatak and K. L. Chopra, Mat. Res. Bull., 23 (1988) 1693. Urea is used in R. S. Liu, C. T. Chang and P. T. Wu, Inorg. Chem., 28 (1989) 154.
B. C. Bunker, J. A. Voight, and H. D. Doughty, "High Temperature Superconducting Materials: Preparation, Properties and Processing", (W. E. Hatfield and J. H. Miller, Ed.), M. Dekker, Inc., New York, 1988, describe the achievement of simultaneous precipitation in a flow system using tetramethylammonium hydroxide and CO.sub.2. Extensive washing was avoided and good stoichiometry achieved. However, the reagent used is expensive.
Attempts have been made to use inexpensive precipitation agents such as alkalis. These attempts have been largely unsuccessful primarily because of lack of proper pH control and solubility differences among the cations lead to the redissolving of the components during washing and aging.
In addition, previous attempts resulted in the retention of unacceptably high amounts of the alkalis, the presence of which degrades the effectiveness of the product superconductor and may even prevent it from achieving the superconducting state.
Two examples of such attempts can be found in I. Legrand, M. Maric, D. Luss, and J. T. Richardson, "Preparation of the 1-2-3 Superconductor by Hydroxide Coprecipitation Route", submitted for publication, and A. M. Kini, U. Geiser, H. C. I. Kao, K. D. Carlson, H. H. Wang, M. R. Monaghan, and J. M. Williams, Inorg. Chem., 26 1834-36 (1987).
Kini et al. developed a procedure in which a 1-2-3 nitrate solution is neutralized to a pH of from 7 to 8 with KOH. Potassium carbonate solution is then added and the precipitate centrifuged and washed with water adjusted to a pH of 9.7 to avoid loss of barium. This technique introduces inhomogeneities in the precipitate. Addition of KOH to the nitrate solution may also result in formation of basic nitrates of copper, which later dissolve to change give nonstoichiometry. In addition, yttrium hydroxide precipitates early. Unless care is taken to stir the solutions well, addition of the carbonate to the nitrate also produces uneven precipitation in the high pH regions of the mixture.
Thus, many of the presently known coprecipitation techniques share, to some degree, the disadvantage of using an expensive precipitation agent. Other techniques lead to the retention of excessive amounts of foreign material in the precipitate which leads to a degradation of the superconductor properties. Many of the known techniques are wasteful of the metal components. This would lead to less than optimal commercial operations when the process is scaled up.
It would be desirable to have high temperature superconducting materials, in particular YBa.sub.2 Cu.sub.3 O.sub.6+x, which would have excellent properties and which could be produced by simple operations, using inexpensive reagents and which could be easily scaled up to commercial continuous operations.
It would also be desirable to have a process for making high quality high temperature superconductors using coprecipitation wherein the conditions are controlled to give products of the stoichiometric ratios with little or no loss of cation.
It would be further desirable to be able to wash the precipitated precursor gel in such a manner as to prevent the loss of cations and at the same time to greatly reduce or eliminate contaminants from the product.
Specifically, there is a clear need for high quality superconducting materials which can be made by a coprecipitation process which is simple, which uses inexpensive reagents, and which can easily be scaled up to a full-scale commercial process which is easy to implement and operate and capable of continuously producing the desired superconducting product.